1. Field of the Invention
This invention pertains in general to a synchronization method and apparatus for digital receivers and, more particularly, to a method and apparatus for obtaining symbol, frame, and carrier synchronization for guard interval-based OFDM signals.
2. Description of the Related Art
Digital audio broadcasting ("DAB") and digital video broadcasting ("DVB") systems provide high quality signals with minimal impact due to multipath fading, noise, or signal drop-offs relative to those of analog broadcasting. The reduction of impact from multipath fading is largely due to the fact that many DAB and DVB systems have adopted orthogonal frequency division multiplexing ("OFDM") modulation. Multipath fading, together with intersymbol interference, may be additionally reduced in OFDM signals by the insertion of guard intervals. For the most part, an OFDM signal is preceded by a cyclic extension of the last portion of a symbol, and this extension is placed as the guard interval.
FIG. 1 shows an OFDM signal having a guard interval, wherein the duration of the guard interval is represented by T.sub.g and the duration of the useful OFDM symbols is represented by T.sub.u. Also, the duration of the guard interval is equal to, for example, approximately one-fourth of the duration of the useful OFDM symbols. The definition of a "useful symbol" is described below. Additionally, the OFDM signal shown in FIG. 1 is the same as that of the European-based Eureka 147 DAB system because the guard interval of that system is also equal to approximately one-fourth of the duration of the OFDM symbols.
FIG. 2 illustrates a transmission frame of the Eureka 147 system. As shown in FIG. 2, the Eureka 147 transmission frame is divided into a synchronization channel, a fast information channel, and a main service channel. The channels that constitute the transmission frame consist of a sequence of consecutive OFDM symbols and each symbol consists of a set of equally-spaced carriers. In addition, as already described, each of the channels is preceded by a periodic extension of the symbol itself that constitutes the guard interval. The duration of the guard interval is approximately 1/4 of the useful symbols' duration, and the symbols in the guard interval are reproduced from the final 1/4 of the useful symbols of the channel.
The synchronization channel always occupies the first two symbols of the transmission frame. The first symbol of the synchronization channel is a null symbol, which is also the first symbol of the transmission frame. The null symbol carries no information, and the DAB signal is primarily noise for the duration of the null symbol. The second symbol of the synchronization channel, which is also the second symbol of the transmission frame, is a phase reference ("PR") symbol that acts as a pilot symbol for the next symbol. The synchronization channel therefore serves as the reference for the time information carried in the fast information channel, and is repeated at the transmission frame rate.
The PR symbol acts not only as a reference for differential quadrature phase shift key modulation and demodulation for the next OFDM symbol, but also as a pilot symbol for synchronization. As with many communication systems, however, synchronization for a DAB or DVB system at various communication layers, especially frame, carrier, and clock synchronization at the physical layer, is important. Because of various changes in propagation channels for DAB and DVB systems, symbol and carrier synchronization are difficult. Although algorithms and systems for conventional OFDM systems have been devised, they usually require different frame structures or do not provide for frame, carrier, and clock synchronization.